ttools: publicise Outliner and Stamper implementation classes.

These classes were previously hidden away in ShapeForm and ShapeMode
implementations.  Publicising them does make the public API a bit
harder to read, but allows to generate ShapeStyle instances under
full programmatic control, without using ConfigMaps.

At some point ShapeMode/ShapeForm should probably get split into a
whole subpackage, then it will look less fiddly.
ttools: provide more control over plot image caching.

In PlotDisplay and its factory constructor in Plot2Task, provide options
to indicate that the data and plot image should or should not be cached
(for instance according to whether the data in the DataStore will/will not
change).
ttools: add ColumnDataSpec

DataSpec implementation intended for programmatic use, when you have
the user coordinates in as columns of a table.